Field of the Invention
The invention relates to an arrangement of optical elements for focusing approximately collimated beams.
Description of the Background Art
Optical elements for beam focusing are known from the prior art, which break collimated input beams around an optical axis into output beams, which overlap in a focus area. Thus, it is possible to convert a comparatively low average inlet irradiance, which is however distributed over a relatively wide entrance pupil, into a comparatively higher average outlet irradiance, which is however concentrated in a relatively narrow cross section of the focus area. From the prior art, the use of such optical elements is known to exceed a minimum irradiance solely within the focus area, above which certain physical effects such as polymerization, optical breakthrough or a melting of solid materials are triggered. Thus, machining of materials or biological tissue is possible in a spatial sector which is relatively sharply limited by the focus area.
Optical elements are advantageous both for achieving particularly high radiation levels as well as for particularly precise machining, with which an input beam of a predetermined cross-section can be focused on a very narrow focus area. For example, for this purpose aspheric lenses are known from the prior art, which can be formed on the basis of the laws of ray optics in such a manner that any input beams for the light of a wavelength extending parallel to the optical axis are refracted into output beams, which intersect at a focal point, which is located at a distance of the focal length from the outlet-side principal plane of the aspheric lens on the optical axis.
Considering wave optical effects, however, no focal point of infinitesimally small extent can be obtained even with such an aspheric lens and also for monochromatic light, but instead only a focus area of finite extent, which is usually indicated by the diameter of the Airy disk,
            d      Airy        =          1.22      ·              λ                  n          ·                      sin            ⁡                          (              α              )                                            ,wherein λ is the wavelength of the monochromatic light, n the refractive index of the medium surrounding the lens, and α the outlet-side half opening angle of the aspheric lens.
Arrangements and methods according to the prior art reduce the extent of the focus area by enlarging the numerical aperture. If the cross section of the entrance pupil remains constant, an increase in the numerical aperture according to the prior art can be achieved by reducing the focal length, thus also reducing the working distance between the front surface of the lens and the material to be machined. According to the prior art, an increase of the numerical aperture can also be achieved by using an immersion fluid which has a higher refractive index than air, between the lens and the material to be machined.